Drilling fluids have been used since the very beginning of oil well drilling operations in the United States and drilling fluids and their chemistry have been and remain an important area for scientific and chemical investigations. The use and desired properties of drilling fluids are comprehensively reviewed in recent U.S. Pat. Nos. 6,339,048 and 6,462,096, issued to the assignee of this application, the entire disclosures of which are incorporated herein by reference.
A drilling fluid is a thixotropic system and exhibits low viscosity when sheared during cutting of the hole into the ground, during agitation and circulation but, when such shearing action is halted, must quickly thicken to among other things hold the “cuttings” from the drill hole in place without sinking. The fluid must therefore become thick rapidly, reaching sufficient gel strength before such suspended materials fall any significant distance. Importantly, this behavior must be totally reversible at all temperatures encountered in the borehole. In addition, even when the drilling fluid is free flowing, it must retain a sufficiently high viscosity to carry all cuttings and other particulate matter from the bottom of the hole back up to the surface.
Since the end of the second World War, hydrocarbon drilling for exploratory and production wells has increasingly been done from platforms located in water settings, often called off-shore drilling. Such fresh and salt water drilling employs floating barges and rigs often fixed in some fashion to the submerged surface of the earth.
Economic and technical advances have recently pushed these drilling operations into deeper waters. Although advances in equipment and engineering have yielded technology capable of drilling in water depths up to 10,000 feet or more, advances required in drilling fluid technology have lagged.
A major problem with oil-based drilling fluids in deepwater drilling is rheological additive temperature sensitivity over the temperature range encountered. During circulation, the drilling fluid typically reaches bottom hole temperatures of about 60 to 80° C. followed by cooling to lower than 5° C. in the riser during its travel upward (due to the inherent low temperature of sea water far below the ocean surface). For successful deepwater drilling, the mud needs to simultaneously suspend the solids and remain pumpable with proper viscosity over these wide temperature ranges.
Drilling fluids thickened with conventional organophilic clay rheological additives particularly suffer considerable viscosity build as the drilling fluid is cooled from a temperature of 60 to 5° C., for example. As a result of this viscosity increase, the drilling fluid, when it reaches low temperatures, is more difficult to pump, the equivalent circulating density (ECD) is increased and losses to the formation (lost circulation) frequently increase.
The requirements for drilling fluids with enhanced temperature properties have also become more complex over the past two decades as a result of changes in directional drilling technology, in which a well is drilled at an angle other than vertical. Such wells are widely known as deviated wells.
Methods for drilling deviating wells have changed greatly over recent years with the production of more powerful and reliable downhole motors, and the invention of more accurate methods utilizing wireline techniques as well as the highly computerized downhole, sensing and micro reduction equipment, including improvements in sounding apparatus and microwave transmission. These techniques permit the instantaneous acquisition of data relating to down-hole conditions without the need to remove the drill string and in fact mean that holes can, and are, drilled at ever increasing lengths.
The advantages of directional drilling include (1) directional drilling allows tapping of fields which cannot effectively be reached by vertical drilling; (2) such drilling permits the use of more economical land-based equipment to explore the immediate off-shore environment; and (3) such drilling make possible the drilling of multiple wells up to several miles from one another, sharing the cost of a single site. In addition, in certain geological formations, increased production can be achieved by deviating the well off-vertical so as to facilitate perforation and development of a narrow producing zone, or redevelopment of a depleted formation.
Use of a downhole motor allows the hole to be deviated by the introduction of a fixed offset or bend just above the drill bit. This offset or bend can be oriented by modern MWD systems which are capable of reporting accurately the current bit and toolface hole angle and azimuth (i.e. the orientation with respect to the upper portion of the hole). It is accordingly possible to rotate the drill string until the toolface has achieved the desired direction of deviation, and then to fix the drill string in place and commence the deviation by starting the motor to extend the hole in the desired deviated direction.
There are, however, a number of inherent problems in the use of directional drilling, which affect the requirements of a drilling mud; namely:                As in deep water drilling, increased ranges of temperatures are encountered.        The annulus carrying the mud to the surface is no longer vertical and extends to far greater distances versus vertical wells.        Gravity on a horizontal hole pulls cuttings, weighting material and particulate matter, not controlled by the drilling fluid, to the bottom side of the bore (not the bottom of the hole as in traditional drilling) and results in drag on the bore wall.        The amount of drilling mud required is increased since the distances are greater, and the time required for the mud to reach the earth's surface also increases.        Curves and kinks in the hole's direction can accumulate cuttings and additives.        
In order to obviate or mitigate these problems, which can cost oil and gas companies millions of dollars per hole, it is an object of the invention to provide drilling fluids with rheological properties particularly appropriate for directional drilling including the improved viscosity stability with temperature discussed above.
For background, it has been long known that organoclays (also called organophilic clays) can be used to thicken drilling fluids. See the very early article by the employee of the assignee hereof J. W. Jordan, “Proceedings of the 10th National Conference on Clays and Clay Minerals” (1963), which discusses a wide range of drilling applications of organoclays from high polarity liquids to low polarity liquids.
Previously mentioned U.S. Pat. No. 6,462,096 discloses oil-based invert emulsion drilling fluids that provide more stable drilling fluid viscosity and anti-settling performance over varying temperatures when compared to conventional fluids containing organoclays.
Patents of the prior art that show developments related to either drilling fluids or chemistry of additives include the following:
U.S. Pat. No. 3,514,399 teaches the use of a mixed dimer acid-monocarboxylic acid salt of an imidazoline in a drilling fluid.
U.S. Pat. No. 5,260,268 describes a product introduced into a well borehole which encompasses water-based drilling fluids and shows a composition comprised of a polycarboxylic acrylating agent reacted with an amine-terminated polyethylene of a molecular weight average from 600 to 10,000. While ethoxylated amines are discussed as a surfactant which may be used in conjunction with the composition, there is no teaching of applications in an oil-based invert emulsion drilling fluid.
U.S. Patent Application Publication No. 2001/0009890 shows an invert emulsion suitable for drilling a subterranean well which uses an ester of a C1 to C12 alcohol and a C8 to C24 monocarboxylic acid—Ethomeen C/15 can be used as an agent in the invention described in the application.
U.S. Pat. No. 5,536,871 issued to the assignee hereof describes a rheological additive which comprises the reaction product of a polyalkoxylated nitrogen-containing compound such as polyoxyethylene (5) cocoalkylamine, a polycarboxylic acid including dimer acids and a liquid diamine.
U.S. Pat. No. 5,610,110 also issued to assignee hereof shows an improved drilling fluid containing a reaction product of an alkoxylated aliphatic amino compound and an organic polycarboxylic acid and a clay based organoclay.
U.S. Pat. No. 5,909,779 at Col. 4, lines 55 to Col. 5, line 15 contains a large laundry list of surfactants, wetting agents and viscosifying agents conventionally used in oil-based drilling fluids including fatty acids, polyamines, imidazoline derivatives and polycarboxylic acids and soaps of fatty acids.
Recent Dow Chemical Company U.S. Pat. No. 6,291,406 describes a well treatment fluid using an amine surfactant to provide a sufficiently stable emulsion. Ethomeens are discussed, particularly bis(2-hydroxyethyl) cocamines and oleyamines.
Commercial rheological drilling fluid additives presently available on the market, however, tend to have increased viscosity while the fluid temperature is low, requiring increased pump pressure which in turn causes increased wear of the drilling gear. Increased pumping horsepower becomes necessary to pump drilling muds through long distances, and increased down-hole pressure under pumping conditions increases fluid loss, fracturing and damage of the formation. Prior art methods of reducing drilling fluid viscosity are not satisfactory because the resultant drilling fluids fail to maintain adequate suspension characteristics when the fluid temperature changes, for example, at down-hole conditions.
There is clearly an unfilled need which has been growing in the past decade for drilling fluids that are able to maintain a relatively consistent rheological profile over a wide temperature range; it is believed that the below unexpected described invention fills this need.